D.D.
Well, I have a slightly different situation. My children have a clotting disorder so they are unable to play contact sports BUT I have established a house rule that no extra-curricular activities could be started until age 7 or second grade. I think that children need to get used to school first and that many families over-schedule their children. My oldest son is a dancer and he has taken several types of dance but mostly taps. My 2nd son competes on a bowling league. My youngest son has a mild learning disability and I would like him to concentrate on school a while longer before starting anything else during the school year. He has participated in some summer activities before, after he turned 7, of course!
I do wonder what will happen to the children that start the strenuous activities young. I've heard that gymnasts have very hard time with their joints and muscles later on in life since they start so young and are pushed so hard at such a young age. It wouldn't surprise me too much if they find similar findings in other sports such as football or wresting since children are starting these types of sports younger and younger.